1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a multifunction peripheral, a printer, and a facsimile apparatus.
2. Description of the Related Art
In this type of image forming apparatus, a latent image is formed on a surface of a latent image support having a moving surface, a toner is applied onto the latent image to obtain a toner image, and the toner image is transferred onto a recording member supported on a surface of a surface moving member, or the obtained toner image is transferred onto the surface of the surface moving member and then the toner image on the surface moving member is transferred onto a recording member. Such image forming apparatuses are known to obtain color images by overlapping monochrome images in plural different colors with each other. Such color image forming apparatuses are demanded to perform operations with higher image quality and higher speed. For example, tandem type image forming apparatuses employing a direct transfer method, which forms a color image on a recording member by transferring monochrome images of black (K), yellow (Y), magenta (M), and cyan (C) formed on respective photosensitive body drums (latent image supports) so as to overlap each other onto the recording member supported and transferred by a recording member transfer belt (surface moving member), are known as color image forming apparatuses that can achieve these demands.
In the tandem type image forming apparatuses employing the direct transfer method, a color shift visible to a user may be generated, when transfer positions of the monochrome images are relatively misaligned on the recording member. When such a color shift is generated, for example, image quality is degraded in a such manner that a thin line image, which is formed by overlapping plural monochrome images with each other, is blotted, or a periphery of an outline of a black text image, which is formed in a background image formed by overlapping plural monochrome images with each other, is not colored. Moreover, what is called a banding phenomenon, which is a color density variation periodically generated in a band form, is generated in a colored background area.
Further, there are also known tandem type image forming apparatuses employing an intermediate transfer method, which forms a color image on a recording member by transferring monochrome images of black (K), yellow (Y), magenta (M), and cyan (C) formed on respective photosensitive body drums (latent image supports) so as to overlap each other on an intermediate transfer belt (surface moving member), and then transferring the color image formed on the intermediate transfer belt onto the recording member. In such a tandem type image forming apparatus employing the intermediate transfer method as well, in a similar manner to the tandem type image forming apparatuses employing a direct transfer method, a color shift visible to a user may be generated, when transfer positions of the monochrome images on the intermediate transfer belt are relatively misaligned.
The color shift visible to a user as described above is caused mainly because surface moving speeds of the respective photosensitive body drums periodically change, causing the transfer positions of the monochrome images on the photosensitive body drums to be relatively misaligned. The periodic changes of the surface moving speeds of the photosensitive body drums become notable when rotational angular speeds of rotational driving forces transmitted to the photosensitive body drums change. The changes of the rotational angular speeds are a transmission error (due to eccentricity of a gear, a cumulative pitch error, and the like) of driving transmission systems provided about shafts of the photosensitive body drums, a transmission error (due to a shaft inclination or a shaft misalignment) caused by a coupling provided so that the photosensitive body drums can be detached from the driving transmission systems, and the like.
An image forming apparatus disclosed in Patent Document 1 is known to suppress such a periodic change in the surface moving speed of the photosensitive body drums to correct a color shift. This image forming apparatus detects the periodic change in the surface moving speed of the photosensitive body drums and finely controls the respective rotational angular speeds of the photosensitive body drums to suppress the periodic changes in the surface moving speeds of the photosensitive body drums. Specifically, plural detection patterns (toner images) formed on the respective photosensitive body drums are transferred sequentially (in the order of K, Y, C, and M) to be aligned in a line onto an intermediate transfer belt. These detection patterns are sequentially detected by a first detection unit. In response to a detection signal of the first detection unit, a periodic change component of the surface moving speeds (detected data) of the photosensitive body drums is detected. The respective rotational angular speeds of the photosensitive body drums are finely controlled so as to cancel out the periodic changes of the surface moving speeds.
Another image forming apparatus capable of correcting such a color shift is disclosed in Patent Document 2. This image forming apparatus has, about the rotation shafts of the photosensitive body drums, plural projecting parts to be detected, which are arranged annularly, and revolve and move in accordance with the rotation of the respective photosensitive body drums. The respective photosensitive body drums have detectors (detecting units) for detecting the passing by of the parts to be detected. The detector detects the parts to be detected which pass through a detection area in accordance with the rotation of the photosensitive body drum, detects a periodic change component of the rotation speed of the photosensitive body drum, which has the same periodicity as one rotation period of the photosensitive body drum, and finely control the respective rotational angular speeds of the photosensitive body drums so as to cancel out the periodic rotation changes.
[Patent Document 1] Japanese Patent Application Publication No. 10-78734
[Patent Document 2] Japanese Patent Application Publication No. 2005-312262
However, the method to finely control the photosensitive body drum, disclosed in Patent Document 1, requires a control step of forming plural detection patterns, detecting these detection patterns, and calculating a control value (correction value) based on the detection signals. This control step is normally performed only once after the photosensitive body drum is assembled in the image forming apparatus. However, a driving transmission part such as a gear and a shaft bearing is deformed by abrasion, or a housing supporting the driving transmission part is deformed by a temperature change, a change of a setting location of the image forming apparatus, and the like, while operating in the market. Thus, the surface moving speeds and the rotation speeds (referred to “speed change” hereinafter), having the same period with one rotation period of the photosensitive body drum, change. When a gear of the photosensitive body drum shaft is inclined due to the abrasion of the shaft bearing and the deformation of the housing, in particular, a speed change of one rotation period of the photosensitive body drum is increased. As a result, a color shift is generated.
To suppress such a color shift, the control step is required to be performed after starting operation in the market as well. In this case, however, there is a problem in that toner is consumed to form the detection patterns. Detection patterns for plural number of rotations of the photosensitive body drums are preferably formed to remove a noise component from pattern detection data and detect a speed fluctuation which changes due to the environment or over time. In this case, therefore, toner consumption is increased.
By the method disclosed in Patent Document 2, by which driving of the photosensitive body drums is finely controlled, there is a problem in precision when positioning the plural parts to be detected. An error of setting the positions of the parts to be detected leads to an error in detecting a speed fluctuation having the same period as one rotation period of the photosensitive body drum. Further, when providing a member formed of plural parts to be detected, which are annularly integrated, to a photosensitive body drum shaft, eccentricity caused by an error in the assembly also leads to an error in detecting a speed fluctuation having the same period as one rotation period of the photosensitive body drum. Such an error in detection not only cannot suppress a color shift, but it may actually increase a color shift. Since there is such a problem of the precision in positioning of the parts to be detected, it has been difficult to use a low cost part to be detected having a relatively dissatisfactory precision of setting position, such as one formed inexpensively by plastic molding or the like in a flange part on a side surface of a photosensitive body drum or a flange part on a side surface of a gear. Therefore, it has been required to use an expensive part to be detected, such as one formed with high precision by edging a metal disk member, to solve the problem of the precision in positioning of the part to be detected.